The present invention relates to a color image forming apparatus.
Heretofore, there has been a color image forming apparatus wherein a laser beam scanning unit is housed, and an image carrier is caused to make several turns to form thereon toner images respectively for Y, M, C and K and these toner images are superposed to form a color image. This color image forming apparatus has been inexpensive because it contains only one laser beam scanning unit, but it has been time-consuming for forming a color image because it has been necessary for the image carrier to make several turns. Under this situation, there is available an image forming apparatus which houses therein plural laser beam scanning units for shortening a period of time for forming a color image. In this image forming apparatus, a color image can be formed in a short period of time because toner images respectively for Y, M, C and K are formed simultaneously in the same time zone through scanning of plural laser beams conducted by plural laser beam scanning units, and these toner images are superposed on an image carrier or on a recording medium.
In an image forming apparatus having therein plural laser beam scanning optical units, when a position of a latent image formed by the laser beam scanning optical unit is relatively deviated, image quality troubles such as shear in colors and shade variation are caused. Though there is considered a device wherein the position deviation mentioned above is automatically adjusted through bending, tilting and parallel movement conducted by a position changing unit utilizing a mirror, a lens and a motor in the laser beam scanning optical unit or by a position changing unit utilizing a dimensional change caused by thermal expansion, there are caused problems that an apparatus has to be made larger or the cost therefor has to be high. Further, there are several types for the position deviation, and even the direction thereof is roughly divided into two types including the primary scanning direction and the sub-scanning direction, and each of them is further subdivided into total magnification and partial magnification. Among these, the magnification in the primary scanning direction is relatively easy to control because it can be controlled electrically as is corrected by changing a dot clock. However, for the curved scanning line and relative deviation of parallelism, it has been necessary to make the complicated control mentioned above.
The first object of the invention, therefore, is to provide a scanning optical unit and an image forming apparatus wherein plural color images formed by plural laser beams emitted from plural laser scanning optical units are not deviated in the sub-scanning direction when they are superposed on an image carrier or on a recording medium, and thereby an appropriate color image with high quality can be obtained.
In the internal arrangement of a color image forming apparatus, it is common that a drum-shaped photoreceptor representing an image carrier is arranged at the central portion, an exposure system and a developing unit both representing an image forming means are arranged to surround the photoreceptor, a transfer means is arranged to be directly under the photoreceptor, and a sheet-feeding cassette is arranged at the lowermost portion in the color image forming apparatus.
Further, it has been attempted to increase a degree of freedom for layout inside a color image forming apparatus by using a belt-shaped photoreceptor (hereinafter referred to also as a belt-shaped image carrier) wherein a photoreceptor layer is provided on a belt-shaped base body trained about driving rollers, in place of the drum-shaped photoreceptor.
For example, there has been known a color image forming apparatus wherein each of plural developing units is provided with different color toner, and a latent image for each color is formed on a belt-shaped image carrier by each of plural exposure systems, and toner images each being for each color are developed to be superposed on the belt-shaped image carrier while it makes one turn, thus the toner images are collectively transferred onto a transfer sheet taken out of a sheet-feeding cassette arranged under the belt-shaped image carrier for obtaining a color image. The image forming system employed in the image forming apparatus mentioned above is called a single-rotation superposing system.
For example, TOKKAIHEI 3-80286 discloses a color image forming apparatus of a single-rotation superposing system, wherein a belt-shaped image carrier is arranged to be long sideways, and plural developing units and plural exposure systems representing image forming means are arranged under the belt-shaped image carrier. In the exposure system in this example, light emitted from a light-emitting diode is converged by a SELFOC lens so that an image is formed on the belt-shaped image carrier.
TOKKAIHEI 4-102874 discloses a color image forming apparatus of a single-rotation superposing system, wherein a belt-shaped image carrier is arranged to be long sideways, plural developing units are arranged under the belt-shaped image carrier, and plural exposure systems representing image forming means are arranged under the developing units. In this example, the exposure system is a scanning exposure system which deflects a laser beam by the use of a rotary polygon mirror.
TOKKAIHEI 5-158319 discloses a color image forming apparatus of a single-rotation superposing system, wherein a belt-shaped image carrier is arranged to be long lengthwise, and plural developing units and plural exposure systems are arranged on both sides of the belt-shaped image carrier. The exposure system in this example is a scanning exposure system.
In the color image forming apparatus described in TOKKAIHEI 4-102874, scanning exposure systems are arranged under the belt-shaped image carrier which is long sideways, and a mirror is arranged to change temporarily a path for the laser beam reflected on the polygon mirror. The mirror is sometimes deformed depending on temperature and humidity to cause distortion. When the mirror is distorted, an electrostatic latent image formed on the belt-shaped image carrier is distorted, and quality of outputted images is sometimes deteriorated, which is a problem. Further, since the scanning exposure systems are arranged under the developing units in this color image forming apparatus, toner sticks to the scanning exposure systems including the mirror, making the accurate exposure to be impossible, and quality of outputted images is deteriorated, which is a problem.
When an exposure system is arranged under a belt-shaped image carrier as in the color image forming apparatus described in TOKKAIHEI 3-80286, it results in the arrangement wherein toner falls and lies on the SELFOC lens due to the exposure direction. If toner falls and lies to interfere with an optical path of a laser beam, there is caused a problem that accurate exposure can be performed and quality of an outputted image is deteriorated. In addition, when comparing an exposure system employing a light-emitting diode array with a scanning exposure system, the exposure system employing a light-emitting diode array has a problem that it is not suitable for high quality outputting, because of the reason that a shape, a size and a position of a light spot forming an image on the surface of an exposure system photoreceptor can not be controlled accurately to the extent of the scanning exposure system.
When a belt-shaped image carrier is arranged to be long sideways as those in the color image forming apparatus described in TOKKAIHEI 4-102874 and that described in TOKKAIHEI 3-80286, there is caused a problem that a large floor space is needed for installation of the color image forming apparatus.
In the color image forming apparatus described in TOKKAIHEI 5-158319, a belt-shaped image carrier is arranged to be long lengthwise, and thereby a space occupied sideways by the belt-shaped image carrier is reduced. However, a floor space needed for substantial installation of the color image forming apparatus is not improved because an exposure system and a developing unit are arranged on both sides of the belt-shaped image carrier. Further, since the exposure system and the developing unit are arranged on both sides of the belt-shaped image carrier, parts of the exposure system and the developing unit can not be used in common for the reason that the direction of rotation of a rotary polygon mirror needs to be reversed, and it leads deviation of the exposure scanning position, which is a problem. Further, due to the decentralized exposure systems, position deviation between exposure means caused by fluctuation of an environmental condition such as heat tends to occur.
Therefore, the second object of the invention is to realize a color image forming apparatus wherein deterioration of image quality is prevented, by reviewing the internal structure of the conventional color image forming apparatus, reducing a floor space by improving efficiency for using a space in the apparatus, reducing toner sticking to exposure systems, and by preventing position deviation between exposure means caused by environmental fluctuation.
In the color image forming apparatus having therein a plurality of laser scanning optical units, color images are formed at high speed through simultaneous scanning of plural laser beams conducted by the plural laser scanning optical units. As stated above, the color image forming apparatus, on the other hand, is required to form a shear-free color image wherein plural toner images (Y, M, C and K) are superposed without being deviated. Therefore, in the color image forming apparatus having plural laser scanning optical units wherein the laser scanning optical units are supposed to be arranged keeping their accurate intervals, there has been a possibility that each laser scanning optical unit is subjected to thermal deformation caused by heat from a fixing means installed in the color image forming apparatus and thereby positions of the laser scanning optical units are shifted. In the case of an image forming apparatus wherein a fixing means is located in the vicinity of the end portion of a belt-shaped image carrier, there is a possibility that heat generated from a fixing unit creeps to a laser scanning optical unit, causing thermal deformation of a lens system and a mirror system, and a problem of deterioration of image quality is caused. This problem is more conspicuous in an image forming apparatus wherein a laser scanning optical unit is located directly above a fixing unit. The same problem lies also on a color image forming apparatus having a belt-shaped transfer means which is for transferring a toner image formed on an image carrier onto a transfer sheet.
Therefore, the third object of the invention is to provide an image forming apparatus capable of forming a shear-free color image with high quality, by preventing thermal deformation caused by heat from a fixing unit and by preventing position deviation of each unit at plural laser scanning optical systems in the image forming apparatus.
In an image forming apparatus wherein an image is formed on an image carrier by a rotary polygon mirror which deflects a laser beam, the laser beam is deflected by the polygon mirror, and then is deflected a few times through mirrors. The reason for this is to arrange a writing unit having therein the rotary polygon mirror at an appropriate position and thereby to make the entire apparatus small. However, even when the entire apparatus can be made small, there still is an occasion where it is not preferable to make the entire apparatus small, from the viewpoint of image forming. In that occasion, the scanning position for a laser beam on an image carrier is required to be set strictly, and in the example of the occasion, plural writing units are installed in an image forming apparatus, and a laser beam emitted from each writing unit is required to scan at an appropriate position on the image carrier. The reason is that when each of scanning positions of plural laser beams is shifted, it is confirmed as deviation on the formed image. Therefore, the system in which a laser beam is deflect with a mirror has not been preferable because a laser optical path is distorted by distortion of the mirror, and there is a possibility that scanning can not be conducted at an appropriate position on the image carrier.
Accordingly, the fourth object of the invention is to provide an image forming apparatus employing plural laser beams to form images on an image carrier through scanning of the laser beams conducted by a rotary polygon mirror wherein a color image with high quality can be obtained by preventing deviation of scanning positions for the plural laser beams on the image carrier and thereby by making the plural laser beams to scan at an appropriate position.
Further, as the speed of color image forming has been accelerated, a laser printer wherein plural writing means each corresponding to each color are provided to be close each other and an LED printer have become available.
With regard to these writing means, each toner image is required to be superposed accurately for obtaining a color image with high quality and therefore each writing means is positioned with high positioning accuracy, because a color image is made by superposing toner images formed through writing and developing conducted after the writing on an image forming body.
In the case of a writing means of a laser scanning optical type, however, only enhancement of positional accuracy for mounting the aforesaid writing means is not enough, and there is caused a problem that each member constituting a scanning optical system is affected by thermal expansion, resulting in variation of positional relation of the writing position, and doubling is caused on a toner image to be formed and image quality is lowered, because heat generating bodies such as a motor to drive a laser beam deflecting means and drive control circuits are housed.
After solving the aforesaid point for improvement, therefore, the fifth object of the invention is to an image forming apparatus wherein thermal expansion caused by temperature rise of each writing means can be controlled by efficient arrangement of extremely simple units, and thereby accuracy of each writing position can be maintained.
Many methods and apparatuses have been proposed for obtaining a color image by the use of an electrophotographic method. For example, as disclosed in TOKKAISHO 61-100770, there is a method wherein forming of latent images corresponding in terms of number to the number of separated colors of a document image and developing therefor are conducted on a photoreceptor drum representing an image carrier, then, developed images are transferred onto a transfer drum for each developing to form a multi-color image on the transfer drum, and thus, a color copy is obtained by transferring onto a recording sheet. It is not avoidable for an apparatus of this method to be large in size and complicated in structure, because a transfer drum having a circumferential surface in a size on which an image equivalent to one sheet can be transferred is required to be provided in addition to a photoreceptor drum.
Further, as is disclosed in TOKKAISHO 61-149972, for example, there is a method wherein forming of latent images corresponding in terms of number to the number of separated colors of a document image and developing therefor are conducted on a photoreceptor drum, and a color copy of a multi-color type is obtained by transferring developed images onto a transfer material for each developing. In this method, it is difficult to superpose multi-color images accurately, and it is impossible to obtain a color copy with excellent quality.
There is further a method wherein a color image is obtained by transferring after superposing color toner images on a photoreceptor drum by repeating forming of latent images corresponding in terms of number to the number of separated colors of a document image and developing therefor with color toners. The basic process of this multi-color type is disclosed in TOKKAISHOs 60-75850, 60-76766, 60-95456, 60-95458 and 60-158475.
In the multi-color image forming apparatus wherein a color image is obtained through the superposing operations stated above, a charging means, an exposure means and plural developing units each housing color toner of different color are arranged around a photoreceptor drum, and a color image is obtained by rotating the photoreceptor drum several times and thereby by developing latent images on the photoreceptor drum, or, plural charging means, plural exposure means and plural developing units each housing color toner of different color are arranged around a photoreceptor drum, and a color image is obtained by conducting charging, imagewise exposure and developing several times and thereby by superposing plural toner images on the photoreceptor drum while the photoreceptor makes one turn.
For example, the structure shown in FIG. 32 can be considered as that for a color image forming apparatus employing a photoreceptor drum. In the image forming apparatus shown in FIG. 32, there are arranged imagewise exposure units 103, 109, 115 and 121, developing units 105, 111, 117 and 123, and charging units 107, 113, 119 and 125, around photoreceptor drum 102 for the purpose of forming toner images each being for Y (yellow), M (magenta), C (cyan) and K (black). However, in the space inside the photoreceptor drum 102, there are many wasteful spaces, which prevents an apparatus from being made small. Further, when the structure like this is employed, it is difficult to obtain a quality image due to deformation of a lens system and a mirror system caused by heat, and it is necessary to provide duct 129 for heat interception use, because the imagewise exposure means 103 is arranged above fixing unit 127 which is a heat source. In addition, the direction of exposure for each of the imagewise exposure units 103, 109, 115 and 121 is different from others and the direction of a developing agent supply section for each developing unit is different from others, which causes a problem that parts can not be used in common, whereby each developing unit can not be structured with the same parts, resulting in an increase in the number of parts.
Though there may be considered the structure wherein imagewise exposure units 203, 209, 215 and 221, developing units 205, 211, 217 and 223, and charging units 207, 213, 219 and 225 are arranged on the circumferential surface on one side of photoreceptor drum 201 as shown in FIG. 33, for avoiding the problem mentioned above, this structure tends to cause problems of a large diameter of the photoreceptor drum 201, further increase of wasteful spaces like those mentioned above, and separation troubles for a transfer material.
With regard to an image carrier, a belt-shaped image carrier wherein photoconductor is coated or evaporated on a flexible belt has also been proposed together with a photoreceptor drum wherein photoconductor is coated or evaporated on the drum circumferential surface as stated above. Since a form of the belt-shaped image carrier (hereinafter referred to also as a belt photoreceptor) is determined when the belt-shaped image carrier is trained about rotary rollers including a driving roller, it is effective when structuring a color image forming apparatus which is made to be compact by utilizing the space effectively.
Since the belt photoreceptor can run along a small curved surface, it has an advantageous point that separation troubles for a transfer material can be prevented by separating the transfer material by utilizing the curved surface, using rotary rollers having a small diameter.
Since it is extremely effective, also for forming a monochromatic image, to separate a transfer material from a belt photoreceptor by the use of rotary rollers having a small diameter, there has also been proposed an image forming apparatus which forms a monochromatic image by using a belt photoreceptor.
In the image forming apparatus employing a belt photoreceptor image forming means such as a charging means, an imagewise exposure means, and a developing means are provided around the belt photoreceptor, and these image forming means face the rotating belt photoreceptor, while being away therefrom by a certain distance.
As a developing means used in a color image forming apparatus wherein toner images are superposed on an image carrier, a developing means conducting non-contact developing is used preferably. The reason for this is as follows, When the developing means conducting contact developing is used, a toner image formed previously is damaged when developing a toner image which is to be formed on the toner image formed previously, in the course of forming toner images by superposing them on a belt photoreceptor.
In the non-contact developing method, a photoreceptor and a developing sleeve which carries thereon developing agents are kept to be away from each other by a certain distance, and DC bias, or bias wherein DC bias and AC bias are superposed is impressed between the photoreceptor and the developing sleeve while maintaining a small clearance through which developing agents sticking to the developing sleeve do not come in contact with the photoreceptor, so that toner may leave a carrier in the developing agents on the developing sleeve to move to a latent image portion for development.
When the non-contact developing method is used, it is necessary to keep the distance between a photoreceptor and a developing sleeve to be constant for the total length of the developing sleeve at considerably high accuracy, and when the distance can not be kept constant, toner images developed turn out to be defective. In the case of a belt photoreceptor, it was extremely difficult to keep the distance between the photoreceptor and a developing sleeve to be constant, because the photoreceptor surface varies as the belt moves.
As a method to solve the problem mentioned above, TOKKAIHEI 3-18868 discloses a proposal wherein a fixed backup plate is provided on the rear side of a rotating belt photoreceptor so that the belt photoreceptor moves along the backup plate, and when stopper members provided at both ends of a developing sleeve are in contact with the backup plate, a prescribed developing clearance is maintained. FIG. 34 is an illustration showing the relation of the foregoing, wherein when stopper members 232A are arranged to be in contact with flat backup plate 40A, even when the axial direction of developing sleeve 55A is not perfectly perpendicular to the moving direction of belt-shaped image carrier 1A, it does not have an influence on developing clearance Dsd formed between the belt-shaped image carrier 1A and the developing sleeve 55A, which is an advantageous point. However, when the belt-shaped image carrier 1A moves, it is not kept to be in close contact with the backup plate 40A, and even when the belt-shaped image carrier 1A is in contact with the backup plate 40A at edge portion 401A of the backup plate 40A, the belt-shaped image carrier 1A is slightly lifted at intermediate portion 402A, and an amount of its rise is unstable, resulting in unstable developing clearance Dsd, thus, satisfactory developing is difficult to be carried out.
TOKKAIHEI 6-51627 also discloses a method to solve the aforesaid problem. In this proposal, a cylindrical backup roller is provided on the rear side of a photoreceptor belt which rotates, and prescribed developing clearance Dsd is maintained when stopper rollers provided on both ends of a developing sleeve are in contact with the backup roller. FIG. 15 is an illustration showing the relation of the foregoing, wherein when backup roller 40B as that stated above is provided on the rear side of belt-shaped image carrier 1B, the rear surface of the belt-shaped image carrier 1B moves while it keeps to be in close contact with the backup roller 40B. Therefore, when a developing means is arranged so that the axial direction of the backup roller 40B and the axial direction of developing sleeve 55B may be positioned to be in parallel with each other perfectly, the developing clearance Dsd can be maintained to be constant as shown in FIG. 35 (a). However, it is actually difficult to keep perfect parallelism, and when the parallelism is not maintained, developing clearance Dsd turns out to be one having different clearances in the axial direction, developing for excellent images is impossible.
Therefore, the sixth object of the invention is to provide an image forming apparatus which solves a problem that it has been difficult to eliminate an influence by fluctuation of the belt photoreceptor surface by keeping the distance between the photoreceptor surface of a belt photoreceptor and a developing sleeve, and is capable of obtaining excellent images by the use of the belt photoreceptor.
In the image forming apparatus, parts such as an image carrier, a charging means and a developing unit need to be replaced when the umber of prints arrives at the prescribed value.
On the other hand, in the case of an image forming apparatus employing a belt-shaped image carrier, an attempt to make an apparatus small by achieving efficient arrangement of parts such as the image carrier has not been made. In the case of a multi-color image forming apparatus, in particular, a plurality of charging means, image forming means and developing means are necessary, and it is unavoidable for the apparatus to be made large.
On the other hand, time to replace the parts which need to be replaced when arriving at the prescribed number of prints varies variously. For example, in the charging means having therein a grid and a charging electrode, life of the grid is shorter than that of the charging electrode. In the case of a multi-color image forming apparatus, a plurality of charging means, image forming means and developing means are necessary as stated above, resulting in many parts to be replaced.
The invention has been achieved in view of the aforesaid problems, and its object is to obtain an image which is excellent and with high quality. Namely, the seventh object of the invention is to provide an image forming apparatus wherein which can be structured to be simple and made to be small in size. Further, the eighth object is to provide an image forming apparatus wherein replacement of parts such as an image carrier or the like is simple. The ninth object is to provide an image forming apparatus wherein replacement of parts such as a developing means is simple.
Further, the tenth object is to provide an image carrier cartridge wherein replacement of parts such as an image carrier or the like is simple. The eleventh object is to provide an image carrier cartridge wherein the mechanism can be simplified and can be made small, and replacement of parts such as an image carrier or the like is simple.
Still further, the twelfth object is to provide an image forming cartridge wherein replacement of parts is simple.
In a color image forming apparatus wherein plural charging means, plural developing means, and plural exposure means are arranged around an image carrier, and a color image is formed by superposing toner images respectively for Y, M, C and K while the image carrier makes one turn, it is necessary to install plural charging means and plural exposure means, which is different from one (hereinafter referred to as "multi-rotation system") wherein a color image is formed by rotating an image carrier many times, and therefore, it is unavoidable for the total size of the color image forming apparatus to be large. In addition, when attempting to install plural charging means, plural developing means and plural exposure means so that they face the image carrier, it is necessary to secure an installation space for each of them along the image carrier surface. As a result, a developed length of an image carrier needs to be established long, and a problem of a large-sized apparatus is further caused. On the other hand, the developing means and the charging means need to be used for the prescribed number of prints and to be replaced at almost the same time, but when plural developing means are installed separately from plural charging means with out being connected to each other, an individual replacement operation is necessary for each of them in the case of replacement, resulting in poor operability for a user.
Therefore, the thirteenth object of the invention is to provide an image forming apparatus having therein plural charging means, plural developing means and plural exposure means all arranged around an image carrier wherein a small space is achieved and a small size of the apparatus can be realized, and to provide an image forming apparatus and an image forming cartridge wherein plural developing means and plural charging means can easily be replaced and operability for a user can be improved.
In order to form images satisfactorily, there have been provided, in an image forming apparatus, some means for detecting the state of the surface of an image carrier (photoreceptor) such as, for example, detecting means including a patch detection sensor which detects an amount of stuck developing agents, a registration sensor which detects the state of registration of images formed by plural developing units, and a surface potential sensor which detects the state of surface potential.
Among these means, the detecting means such as the patch detection sensor and the registration sensor are required to be located at the downstream side of developing means, because they detect images visualized with developing agents on the image carrier.
Since images to be detected by a transfer means are required to be detected before the images are transferred onto a transfer material, the detecting means is located between a developing means and the transfer means. Further, in the case of the registration detection which requires accuracy, it is preferable that the registration sensor is arranged to be as close as possible to the developing means, because more developing agents are scattered at the place which is farther from the developing means.
Incidentally, it can also be considered to separately form an image for detection use outside the image forming area (outside the primary scanning area) on the image carrier and to detect this image. In this case, however, various components such as the developing unit, the transfer means and the cleaning means are required to function even in the outside of the image forming area, which causes problems of cost and a large size of the apparatus.
On the other hand, between the developing means and the transfer means, there is formed a conveyance path through which a transfer material is conveyed to the transfer means. When the detecting means is provided in the vicinity of the image carrier for enhancing accuracy of the detecting means, the conveyance path needs to make a detour around the detecting means. When there is a sharp curved portion in the detoured conveyance path, there is caused a problem of "jamming" of a transfer material, while when the curved portion is formed to be of a gentle curve for avoiding the "jamming", there is caused a problem of a large-sized apparatus. Therefore, how to arrange the detecting means and the conveyance means is an issue to be considered.
Therefore, the fourteenth object of the invention is to provide an image forming apparatus which can be made small by ellaborated arrangement of an image carrier surface state detecting means and a conveyance path.
In an image forming apparatus having a belt-shaped image carrier wherein the belt-shaped image carrier is trained about plural rollers, it is common that the rollers about which the belt-shaped image carrier is trained are made to have a small diameter. For example, in the image forming apparatus described in TOKKAIHEI 3-77975, toner images are transferred onto a recording sheet at the portion of a roller about which a belt-shaped image carrier is trained, and the recording sheet is separated, utilizing a curve of the roller. An enlarged diagram of the transfer section and its surrounding in TOKKAIHEI 3-77975 is shown in FIG. 13. When space 5" of the portion through which transfer body P conveyed by transfer belt 91" advances to a transfer section between transfer roller 5' and transfer electrode 91' is broad as shown in the diagram, movement of toner t from belt-shaped image carrier 1' to the transfer body P is started by an electric field for transfer, before the transfer body P comes in close contact with the belt-shaped image carrier 1'. Therefore, a problem that toner t is scattered as shown with arrow marks in the diagram tends to occur.
In an image forming apparatus having a drum-shaped image carrier, it is relatively easy, due to an elaborated shape of a transfer body guiding member at the guiding side, to make a transfer body to come in close contact with the image carrier before the application of an electric field for transfer, because a size of a diameter of the drum can be secured to a certain level. However, in the case of a roller with a small diameter about which a belt-shaped image carrier is trained, there has been caused a problem that an angle of bending for a transfer body is too great and a sheet to be conveyed is jammed.
Therefore, the fifteenth object of the invention is to provide an image forming apparatus having a belt-shaped image carrier trained about plural rollers wherein toner scattering from a belt-shaped photoreceptor can be prevented, and toner images formed on the belt photoreceptor can be transferred onto a recording sheet properly, making an excellent image to be obtained.
With regard to a general internal structure of an image forming apparatus, there are many examples wherein a photoreceptor representing an image carrier is installed at the central portion and an exposure means, a developing unit and a transfer means are arranged around the photoreceptor to surround it, and the transfer means is arranged directly under the photoreceptor while a sheet-feeding cassette is arranged directly under the transfer means. In an image forming apparatus having the internal structure wherein the transfer means is arranged directly under the photoreceptor and a sheet-feeding cassette is arranged directly under the transfer means, a transfer sheet is conveyed from the sheet-feeding cassette to the transfer means through an S-shaped path or a U-shaped path in many cases, due to the positional relation between the sheet-feeding cassette and the transfer means.
When trying to convey a transfer sheet through an S-shaped path or a U-shaped path, there is a possibility that sheet jamming takes place at a turning portion, and when conveying the stiff transfer sheet, in particular, occurrence of sheet jamming at the turning portion is conspicuous. On the other hand, a normal image forming apparatus is equipped with a manual sheet-feeding mechanism, and a conveyance path based on the manual sheet-feeding mechanism is straight in shape, which reduces occurrence of sheet jamming. However, in the manual sheet-feeding mechanism, a folding tray, for example, is unfolded to be protruded extensively from the image forming apparatus and then a transfer sheet is placed on the tray, which causes a problem that a floor space of the image forming apparatus is considerably increased. In addition, there is caused a problem that sheet size detection is difficult, because the manual sheet-feeding mechanism does not usually have a rear end stopper.
Therefore, the sixteenth object of the invention is to realize a color image forming apparatus wherein an increase of an area for a sheet-feeding unit is controlled and a height of the sheet-feeding unit is reduced, by reviewing the conventional internal structure of a color image forming apparatus, whereby by controlling occurrence of sheet jamming caused in conveying a transfer sheet (for example, transfer sheet used frequently) placed on at least one transfer body holding means, and further by improving efficiency of using spaces in the apparatus.
Arrangement of a sheet-feeding cassette in an image forming apparatus which has been commonly known is shown in each of FIGS. 29 and 30. In the conventional sheet-feeding unit shown in FIG. 29, small-sized sheet-feeding cassette 41a housing therein small-sized sheets is stacked on large-sized sheet-feeding cassette 410b housing therein large-sized sheets, and sheets ejected respectively out of the sheet-feeding cassettes 41a and 41b respectively for both sizes pass respectively through path a and path b, then are turned by 180.degree. and are directed to transfer means 41c. The path a and the path b join each other on the half way. In another conventional sheet-feeding unit shown in FIG. 30, sheets pass through paths c and d which extend respectively from small-sized cassette 41a and large-sized cassette 41b and are almost straight and join each other on the half way, and enter transfer means 41c.
With regard to the sheet-feeding unit shown in FIG. 29, since sheets of both sizes are turned by 180.degree., when the curvature of the turning is made large, there is caused a possibility of sheet jamming. When the curvature is made small to avoid the possibility of sheet jamming, it is unavoidable for distance n in the vertical direction from the bottom surface of the large-sized sheet-feeding cassette 410b to the transfer means 41c to be great, and thereby the total height of the apparatus is increased. With regard to the sheet-feeding unit shown in FIG. 30, a possibility of sheet jamming is small because paths c and d are not turned, but distance m in the horizontal direction from the end of the large-sized sheet-feeding cassette 410b to the transfer means 41c is great, and the total width of the apparatus in the lateral direction is great, which has been a problem.
As stated above, plural sheet-feeding cassettes have not been arranged efficiently in the conventional sheet-feeding unit. Further, there have been available cassettes for exclusive use for small-sized sheets, but for the apparatus capable of printing for large sizes, most of cassettes therefor have been those capable of housing large-sized sheets wherein a size regulating plate inside the cassette is fixed, thus the space occupied by the cassette is large, and a space where no sheets are housed can not be used effectively, which have been problems.
Therefore, the seventeenth object of the invention is to provide a sheet-feeding unit and an image forming apparatus wherein a sheet-feeding cassette housing therein small-sized recording sheets and a sheet-feeding cassette housing therein large-sized recording sheets are arranged efficiently, and thereby a small size of the apparatus is realized, and a recording sheet can be conveyed smoothly.
There has been further known an image forming apparatus wherein a belt-shaped image carrier which has a seam and rotates is used, and toner images are formed continuously on the belt-shaped image carrier to be transferred onto a transfer sheet. Incidentally, it is preferable that a developed length of the belt-shaped image carrier is controlled to be as short as possible, because it has an influence on the size of the image forming apparatus. On the other hand, it is also desired to set the developed length to be long, for the requirement to form an image on the large-sized transfer sheet, or to increase printing speed by forming plural images on the belt-shaped image carrier. In the belt-shaped image carrier having a seam, therefore, it has been necessary, for efficient continuous transferring onto a transfer sheet, to establish the developed length of the belt-shaped image carrier and an effective image area by giving consideration to the length of a transfer sheet and sheet-feeding intervals.
Therefore, the eighteenth object of the invention is to provide an image forming apparatus for forming toner images continuously on a belt-shaped image carrier which has a seam and rotates and for transferring them onto a recording sheet, wherein it is possible, for efficient continuous transferring onto a recording sheet, to establish a developed length and an effective image area of the belt-shaped image carrier by giving consideration to the length of a transfer sheet and sheet-feeding intervals, and to achieve a small size of the apparatus.